Given a choice between someone who promises to fight for the thing you care about, but might be lying, and someone who promises to not fight for the thing you care about, the choice seems easy.
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Someone promising you unreasonable things is much more likely to be fucking you over. Some would say that promising you unreasonable things and then fucking you over is the basic thread of all politics, Trump is just taking it several steps further because apparently people have become unmoored from the reality of government and so cannot judge what is unrealistic anymore.
Or at least he'll say that.
Whether he'll a) change his mind the next day, b) have any feasible plan to implement this, c) whether it's actually a net-positive for his demographic, d) whether he supports a bunch of other things that will hurt this demographic are all less clear.
I mean this is a man who manufactures stuff in China himself (http://abcnews.go.com/Politics/donald-trump-admits-gear-made...).
Why not just increase sales tax and then give it to American union members if buying votes by subsidising them is a good strategy? It's basically the same thing, but it seems a lot less "American" to be honest about it.